1. Field of the Invention
The present invention relates to an optical element, an optical circuit provided with the optical element, and a method for producing the optical element.
2. Description of the Related Art
Due to the rapid spread of the Internet, there is a strong demand for an increase in the information transmission capacity of an optical fiber communication network. Under such a circumstance, wavelength division multiplexing (WDM) has been developed rapidly. The WDM is a communication technique of multiplexing independent information to transmit it, using light having a plurality of different wavelengths. According to this technique, in order to demultiplex a signal, an optical demultiplexer having a satisfactory wavelength selectivity is required.
In recent years, optical communication by the WDM is applied not only to long distance networks but also to short distance, metropolitan and access networks. In this case, Coarse Wavelength Division Multiplexing (CWDM) using a relatively wide channel width is used mainly, instead of Dense Wavelength Division Multiplexing (DWDM) as in long distance networks.
Unlike the DWDM, an optical demultiplexer in the CWDM is required to have performance such as a small setting space and high resistance to temperature and humidity, and also is required to be mass-produced. More specifically, miniaturization, high stability, low cost, and the like are demanded of the optical demultiplexer.
One way to satisfy the above-mentioned demand is to form an optical demultiplexer of a planar lightwave circuit (PLC) type using an optical waveguide. The optical demultiplexer of a PLC type can be miniaturized, and its substrate can be processed in large quantity on a wafer basis by lithography and dry etching. Furthermore, an optical waveguide using silica glass is matched satisfactorily in refractive index with an optical fiber and has a small connection loss, so that the optical waveguide using silica glass is highly practical.
As the above-mentioned optical demultiplexer, arrayed waveguide gratings (AWG) are known. The AWG allows a plurality of optical waveguides (optical waveguide array) having gradually varying optical path lengths to propagate light containing a plurality of wavelength components, and separates wavelengths using a diffraction phenomenon due to a phase shift thus generated.
However, the AWG originally is an optical demultiplexer developed for the DWDM. Therefore, even if the AWG is designed for the CWDM, the effects of cost reduction and miniaturization are small, and hence, the AWG is not suitable for the CWDM.
On the other hand, an optical demultiplexer that separates wavelengths using a reflection-type or transmission-type grating also is well known. This type of optical demultiplexer is configured by combining optical components. An optical demultiplexer in which an optical system is integrated into a PLC also has been developed. Such an optical demultiplexer is described, for example, in “S. Janz and other 13 people, ‘Proceedings of OFC 2002’, (U.S.), 2002, TVK2” and “Christopher, N. Morgan and other 4 people, ‘IEEE Photonics Technology letters’, (U.S.), 2002, vol. 14, no. 9, pp. 1303-1305”.
A PLC using a reflection-type blaze diffraction grating can be miniaturized by tens of % or more with respect to the AWG, and an optical demultiplexer in which a diffraction grating is integrated into a PLC has the potential for the CWDM.
Furthermore, recently, a photonic crystal has been actively studied as a PLC. The photonic crystal has a configuration in which materials with a large refractive index difference are arranged regularly at a period of about a light wavelength. Due to such a configuration, characteristics that are not found in a conventional homogeneous material, such as steep bending of light and complete confinement can be exhibited.
In order to form a photonic crystal, for example, in an optical communication field, a fabrication technique for forming a periodic structure at a scale from a micron to a submicron is required. A slab-type two-dimensional photonic crystal, in which submicron holes and columnar members are arranged on a substrate surface generally can be produced using a submicron patterning apparatus such as an electron beam drawing apparatus and a dry etching apparatus. Thus, forming a submicron periodic structure on a substrate surface is becoming relatively easy with the development of a fabrication technique for a semiconductor.
Furthermore, a photonic crystal having a periodic structure in a vertical direction of a substrate also has been proposed. By using a two-dimensional or three-dimensional photonic crystal having a periodicity in a vertical direction, an optical resonator using a complete bandgap and a polarizer can be formed. More specifically, a production method for irradiating photosensitive polymer resin with a laser in three directions and forming a periodic structure using light interference, a production method for filling a substrate with silica fine particles with high density, a production method for forming a multi-layered film while maintaining a regular concave/convex shape on the surface of a substrate, and the like have been proposed. However, according to most of the above-mentioned production methods, a point defect and a line defect are introduced into a particular position. Therefore, the degree of freedom in terms of configuration is poor.
In contrast, a three-dimensional photonic crystal with a high degree of freedom having a periodic structure in a vertical direction of a substrate has been reported in “Susumu Noda, Katsuhiro Tomoda, Noritsugu Yamamoto, Alongkarn Chutian, SCIENCE, vol. 289, pp. 604-606, 2000”. According to this three-dimensional photonic crystal, the process of attaching periodic structures of line & space formed on a pair of substrates to each other, and peeling only one of the substrates is repeated, whereby a stacked periodic structure (called woodpile type) is realized. According to this production method, a part of a period is removed on a layer basis, or a structure with a varied period can be formed. For example, a photonic crystal waveguide for bending light at a right angle, and a reflection mirror due to a three-dimensional complete bandgap have been reported.
However, in a reflection-type diffraction grating integrated into a PLC, a diffraction grating surface with a height of about several μm from a substrate needs to be formed. Furthermore, a metal film is required as a reflection surface on a diffraction grating surface. In order to produce such a configuration, a complicated and sophisticated processing technique is required.
It also is well known that a wavelength resolution by a diffraction grating is proportional to a product of the order of diffracted light and the number of gratings. Assuming that the order of diffracted light and the size of a diffraction grating are constant, in order to enhance the resolution, the grating period of a diffraction grating must be reduced. In a blaze diffraction grating, when the grating period is about a wavelength, an efficiency difference due to a polarization direction (TE polarized light and TM polarized light), i.e., polarization dependent loss (PDL) is conspicuous. In contrast, when a grating period is increased so as to alleviate the PDL, the number of gratings must be increased, which enlarges a grating. This results in difficulty in miniaturization of an optical demultiplexer. Furthermore, when the order of diffracted light is increased, it is required to remove unnecessary light of a high order using another apparatus, so that an efficiency is reduced necessarily.
On the other hand, in a PLC using a transmission-type diffraction grating, it is required to form a space for forming a diffraction grating in a slab waveguide. However, in the slab waveguide, a cladding layer is formed in an upper portion. Therefore, even if it is attempted to form a diffraction grating in a slab waveguide, it is difficult to maintain a space for forming the diffraction grating when forming a cladding layer in an upper portion of the slab waveguide. Furthermore, air may be used a cladding instead of forming a cladding layer in an upper portion. However, according to such a slab waveguide, a propagation mode becomes a multi-mode, making the polarization dependency very conspicuous.
As the production method for forming a periodic structure, a procedure for forming structures in which defects and the like are freely formed in respective layers, and stacking the structures, so as to form a periodic structure in a vertical direction of the substrate as described above is desirable because of the high degree of freedom.
On the other hand, according to the method for attaching periodic structures formed on a pair of substrates to each other, and peeling only one of the substrates, patterns of new layers are stacked on an underlying pattern. Therefore, not just any patterns may be stacked, and there is a constraint on the patterns to be used. More specifically, the degree of freedom of a structure is not so high.
Furthermore, in order to attach the periodic structures to each other, for example, assuming that a semiconductor is a substrate material, a high-degree and complicated process is required, in which structures are fused to each other in a furnace of 500° C. with a positional precision at nano-level, and repeating this by the number of layers. Needless to say, a material to be used also is limited to those which can be fused. Furthermore, it is not easy to enlarge an area.